The sisters and their brother Thomas grew up in an aristocratic country house with emotionally distant parents and a large household with numerous servants; this family dynamic was not unusual for upper-class families of the time. There was also a disregard for formal education of women of the family, and they were expected to marry at a young age to a financially well-off husband. The children had a private language called "Boudledidge" (pronounced 'bowdledidge'), and each had a different nickname for the others. Their parents were described as "nature's fascists". At least two of their daughters followed in their footsteps; one turned her back on her inherited privileges, ran away to become a communist, a result of the excitement of European politics in the 1930s. Jessica's memoir Hons and Rebels describes their upbringing, and Nancy obviously drew upon her family members for characters in her novels.

Upon the outbreak of the Second World War, their political views came into sharper relief. "Farve" remained a conservative, but "Muv" usually supported her fascist daughters, and they separated in the late 1940s. Nancy, a moderate socialist, worked in London during the Blitz. Pamela remained seemingly non-political, although reportedly a rabid anti-Semite.[5] Tom, a fascist, refused to fight Germany but volunteered to fight against Imperial Japan. He was killed in action a short time after arriving in Asia. Diana, married to Sir Oswald Mosley, leader of the British Union of Fascists, was imprisoned in London for three years under Defence Regulation 18B. Unity, distraught over the war declaration against Germany, tried to commit suicide by shooting herself in the head. She suffered brain damage which eventually led to her early death. Jessica, a communist supporter, had moved to the US, but her husband Esmond Romilly volunteered for the RCAF and died when his bomber developed mechanical problems over the North Sea.[6] In numerous letters Jessica stated that her daughter received a pension from the Canadian government from Esmond's death until she turned 18.[6] The political rift between Jessica and Diana left them estranged until their deaths. The other sisters kept in frequent contact.

The sisters were prolific letter-writers, and a substantial body of correspondence still exists, principally letters between them.[2]

Nancy Mitford (28 November 1904 – 30 June 1973). Married Peter Rodd and had a longstanding relationship with French politician and statesman Gaston Palewski. Lived in France much of her adult life. Writer of many novels, including her most popular (and somewhat autobiographical), The Pursuit of Love and Love in a Cold Climate. Also a noted biographer of historical figures, including the Sun King.

Pamela Mitford (25 November 1907 – 12 April 1994). Known as "Woman." Married and divorced the millionaire physicist Derek Jackson. John Betjeman, who for a time was in love with her, referred to her as the "Rural Mitford". After her divorce, she spent the remainder of her life as the companion of Giuditta Tommasi (died 1993), an Italian horsewoman.[7]

Thomas Mitford (2 January 1909 – 30 March 1945). Educated at Eton, lover of James Lees-Milne there.[8] Regular lover of Tilly Losch during her marriage to Edward James. Died as a soldier in Burma. According to Jessica's letters, he supported British fascism and was stationed in Burma after refusing to fight in Europe.[6]

A fictional family based on the Mitford sisters features prominently in author Jo Walton's 2007 novel Ha'penny; Viola Lark, one of the point-of-view characters, is one of the sisters, another is married to Himmler, and a third is a Communist spy.

The fictional "Combe sisters" in the BBC 2 series Bellamy's People, first broadcast in 2010, bear a striking resemblance to the Mitford sisters. Bellamy meets two of the surviving Combe sisters, said to have been notorious in the 1930s and '40s for their extreme political views, now living together in a strained relationship in the dramatically different political realities of 2010. One an avid fascist and the other a committed Communist, the sisters have hit upon the solution of dividing their stately home down the middle, each converting her side into a homage to her ideology.